The invention relates to a sheet feed apparatus and more especially to a paper feed apparatus suitable for use in a photocopying machine wherein an original to be copied is conveyed, by a gripper engageable with the leading edge of the original, through an exposure station and thence along a rotating supporting surface for the original and is discharged at a delivery station after a predetermined number of revolutions around the supporting surface.
German Offenlegungsschrift No. 2,206,063 discloses a sheet feed device for moving documents to be copied through the exposure and projection regions of a copying machine. At the sheet processing point, the feed device comprises a first conveyor unit in the form of a drum which includes a device for holding the leading edge of a sheet to be conveyed, the sheet being movable by a drive mechanism past the sheet processing point and then being capable of being stopped at a predetermined point. A second conveyor unit, consisting of conveyor belts which are provided in the lower region behind the drum, moves the trailing edge of the sheet being thus conveyed past the processing point after its leading edge has stopped. The conveyor belts are endless belts and frictionally engage a sheet present on the drum and displace it relative to the drum after it has stopped. The device for holding the leading edge of an original is a component of the drum and is integrated with it so that when the sheet is stopped, the whole mass of the drum including the holding device must be stopped. When, after stoppage of the drum, the original is further conveyed between the resilient conveyor belts and the drum, the resulting friction leads to abrasion at the back of the original.
A further disadvantage results from the fact that, when the drum is stopped, a heavy contact pressure must be exerted on the original by the resilient conveyor belts in order to obtain adequate friction for further conveying of the original. Since the frictional force between the surfaces of the conveyor belts and the front of the original counteracts the frictional force between the back of the original and the drum, a jerky movement results in the transition phase, which under some circumstances may lead to a so-called "blurred" or shortened copy, wherein straight lines are slightly offset or shortened at a certain point.
German Auslegeschrift No. 1,902,112 discloses a conveyor device for a photocopying machine wherein the original to be copied is supplied individually from a pile to a supporting surface at an exposure station by means of conveyor belts and subsequently conveyed by conveyor belts for deposit on a delivery pile. For this purpose, a first controllable conveyor device takes the separated original and conveys it to the supporting surface by means of a reversible belt conveyor device which is driven by a reversible drive mechanism. This is controlled by a control circuit including a distance measuring device which can be actuated by a switch responding to the separated original and which produces control signals depending on the measured values corresponding to the distances conveyed. In such an apparatus no provision is made for stopping the leading edge of the original. In order to render possible reproduction of long originals, the sheet feed apparatus must be of considerable length so that a compact form of construction is impossible.
Although both known devices fulfill their task, they do not permit the introduction of a second original during the copying of the first original.